A microphone for transducing a user's speech into an electrical signal is a widely used component in many devices, such as wireless communications devices including cellular phones. The analog output signal from the microphone is typically applied to amplification circuitry for increasing the signal level, and the amplified signal may then be applied to an analog-to-digital converter (ADC) to generate a digital representation of a user's speech, or in general any acoustic signal waveform including music.
As can be appreciated, the amplification circuitry should provide a good signal-to-noise ratio (SNR) so as to accurately and faithfully reproduce the user's speech. However, as devices that contain a microphone become smaller and more compact there is an increased possibility of internally generated noise signals adversely affecting the SNR. In general, as more functionalities use same resources, such as a ground plane, interference cancellation in and due to the ground plane becomes a more important issue.
The following U.S. patents are all illustrative of conventional microphone amplification circuitry: U.S. Pat. No. 4,629,910, “High Input Impedance Circuit”, Early et al.; U.S. Pat. No. 5,097,224, “Self-Biasing, Low Noise Amplifier of Extended Dynamic Range”, Madaffari et al.; U.S. Pat. No. 5,589,799, “Low Noise Amplifier for Microphone”, Madaffari et al.; U.S. Pat. No. 6,160,450, “Self-Biased, Phantom-Powered and Feedback-Stabilized Amplifier for Electret Microphone”, Eschauzier et al.; U.S. Pat. No. 6,218,883, “Semiconductor Integrated Circuit for Electric Microphone”, Takeuchi; U.S. Pat. No. 6,275,112 B1, “Efficient Microphone Bias Amplifier with High Output Voltage/Current Capability and Excellent PSRR”, Muza; U.S. Pat. No. 6,353,344 B1, “High Impedance Bias Circuit”, Lafort; U.S. Pat. No. 6,608,905 B1, “Microphone Bias Current Measurement Circuit”, Muza et al.; U.S. Pat. No. 6,842,525 B1, “Signal Amplification Circuit and Process for Neutralizing Noise from a Power Supply Voltage”, Mellot; and U.S. Pat. No. 6,888,408, B2, “Preamplifier for Two Terminal Electret Condenser Microphones”, Fürst et al. Reference can also be made US 2005/0151589 A1, “Amplifier Circuit for Capacitive Transducers”, Fallesen, and to EP 1 096 831 A2, “Semiconductor Amplifying Circuit and Semiconductor Electret Condenser Microphone”, Takeuchi et al.
The “electret microphone” referred to in several of the foregoing patent documents is widely used type of condenser microphone that has a permanently charged dielectric (electret) between two parallel metal plates (electrodes), one of which is attached to a diaphragm. The diaphragm moves in response to the pressure or particle velocity of sound waves, thereby changing the distance and, therefore, the capacitance, between the diaphragm and its electrode, or backplate. Since the amount of charge is fixed, the voltage between the diaphragm and backplate changes in a manner which is inversely proportional to the change in capacitance. A suitable model for an electret microphone is a capacitor Celectret connected in series with a voltage source Velectret. The electret microphone typically includes an active element such as a FET, and thus requires a source of bias voltage to operate.